[MODEL] Removed the dependency on "activerecord" from the main .gemspec file
[MODEL] Do not require "activerecord" for all tests (in helper)

Instead, require it in each individual file, so we can run it against BUNDLE_GEMFILE from the "gemfiles" folder.

Also, move class definitions into the context, so they are executed only after the test setup -- which sets up the database connection (otherwise, we'd get "connection not established" errors)
[MODEL] Added auxiliary Rake task for integration tests, run against 3 and 4 gemfiles

With the refactored tests, we have to provide a specific BUNDLE_GEMFILE for integration tests,
so added a wrapper Rake task, which sets it up and runs the auxiliary task.

The interface for the end user stays the same, they just run:

    $ bundle exec rake test:integration
[MODEL] Merge branch 'dependency_hell_ice_bucket_challenge' into 'master'